Ultraviolet (UV)-emitting phosphors are used in fluorescent lamp applications for skin tanning where both UVA and UVB radiation is needed. UVA is defined by the U.S. Food & Drug Administration (FDA) as radiation from 320 nm to 400 nm and UVB is defined as radiation from 260 nm to 320 nm. A sensitive parameter for cosmetic, attractive tanning is the ratio of persistent (indirect) to immediate (direct) pigmentation of the skin. In general, UVA radiation mainly induces immediate pigmentation. This results in fast tanning and a grayish-brown color of the skin that disappears after a short time. On the other hand, UVB radiation promotes a long-lasting, reddish brown tanning of the skin. However, prolonged exposure to UVB radiation will also result in severe sunburn. Thus, the majority of the UV flux of tanning lamps is emitted in the UVA region with the balance in the UVB region. This is also intended to generally mimic the relative proportions of UVA and UVB in natural sunlight.
To provide fairly a good tan, a tanning lamp usually produces a controlled amount of UVB for melanogensis and an amount of UVA sufficient to induce immediate pigment darkening. In the present state of the art, this is achieved by blending different UVA- and UVB-emitting phosphors to yield a proper balance of UVA and UVB. The most common UVA-emitting phosphors include: BaSi2O5:Pb, SrB4O7:Eu, YPO4:Ce, and (Ce,Mg)BaAl11O18:Ce. The main UVB-emitting phosphors include: MgSrAl11O17:Ce, LaPO4:Ce, and (Ca,Zn)3(PO4)2:Tl. The relative proportions of the individual phosphor constituents in the lamp depend upon emission efficiency of the individual phosphors and the desired UVA to UVB ratio. Unfortunately, because each UVA- and UVB-emitting phosphor will behave differently over the life of the lamp, the initially formulated UVB to UVA ratio can change over time. This means that the effectiveness of the tanning lamps may also vary over the life of a lamp. Thus, it would be an advantage to have a single-component, UV-emitting phosphor capable of producing a wide range of UVB to UVA ratios.